Field devices, such as radar level gauges are suitably used for measuring the level of products such as process fluids, granular compounds and other materials. An example of such a radar level gauge can include a microwave unit for transmitting microwaves towards the surface and receiving microwaves reflected by the surface, processing circuitry arranged to communicate with the microwave unit and to determine the level based on a relation between transmitted and received microwaves, an interface for connecting said processing circuitry externally of the radar level gauge, and a power management circuitry providing the microwave unit and the processing circuitry with operating power.
In order to ensure a satisfactory signal level of the received echo, the emitted microwaves must have a sufficient power level. The processing of received signals also requires significant power, and in some cases the clock frequency of the processor is increased during the processing in order to enable high speed calculations. In combination, this results in an increased demand of power during certain parts of the measuring cycle. The power requirements are especially high for Frequency Modulated Continuous Wave (FMCW) systems. However, the provision of power is relatively difficult to achieve in practice, since energy is normally a scarce resource in the above-discussed and other field devices.
In particular, limited available power is a problem in systems using a two wire current loop for communication and power supply. Radar level gauges for measuring a level in a tank, and other types of field devices, may often be communicate with a remote location, such as a control room, by means of a two-wire interface, where only two lines serve to both supply the sensor with limited power and to communicate a measurement signal indicative of a process variable determined by the field device. The interface can be a 4-20 mA industrial loop with or without superimposed digital communication, or another two-wire fieldbus, such as Fieldbus Foundation (FF) or Profibus.
A loop-powered field device therefore needs to be capable of both providing a measurement signal to the two-wire current bus, and of drawing power from the two-wire current bus for operation of the field device.
According to one known configuration, a controllable current source and a regulated converter are connected in series along the two-wire control loop. In such a configuration, the measurement signal is provided to the two-wire current bus using the controllable current source, which may be controllable to control the DC current on the two-wire current loop and/or modulate the current to provide digital data, and the regulated converter converts a regulated input voltage to an output voltage used to power the field device.
For various field devices, especially various recent radar level gauges, the configuration described above is not capable of providing sufficient power to the field device under all operating situations/conditions.